Directional solidification of metals and semiconductors requires high temperature to melt the material (typically 1000.degree.-1500.degree. C.) followed by the solidification of the material along a flat solidification surface (front) to achieve desired crystal growth. The solidification front is established by a sharp temperature gradient of a hundred degrees or so (somewhat above and below the melting point). The temperature gradient must then be moved axially along the sample to progressively solidify it. It is important that the gradient be established over the shortest possible distance with 1 inch or less being desired.
Currently two types of furnaces are in use. In one type the temperature gradient is established on the furnace wall and the sample is moved. This is mechanically complex and imposes starting and stopping acceleration on the sample which is undesirable. Therefore, the current preferred approach is to use a segmented furnace with individual programmable heaters. The furnace is covered with insulation and an external water cooling jacket. In operation the coolant is always flowing and the heaters are controlled to provide the desired axial temperature gradient profile.
From a power standpoint this is clearly a brute force approach. The thick insulation cannot be too efficient since individual segments of the furnace must cool rapidly when power is reduced in order to move the temperature gradient at the desired velocity. Therefore, high power is required because of the large (and constant) heat leak through the insulation to the coolant (typically water). The high power consumption of current furnace designs precludes them from large scale application in space where major improvements in material quality are believed obtainable due to the absence of gravitation forces. Such major improvements occur from the elimination of convective flow in the melt at the solidification interface which provides a more homogeneous material.